1. Field of the Invention
The present invention relates to an imaging apparatus having an auto-focus function usable in a digital camera and relates to an auto-focus control method.
2. Description of the Related Art
An auto-focus processing by a contrast detection method is one of typical methods for bringing a subject into focus. The auto-focus processing by the contrast detection method is as follows; (1) a focus lens is driven from a lens end to a lens end within a drivable range of a lens; (2) a contrast component is detected from an imaging signal, which is a CCD output at this time, and (3) its waveform is interpreted, thereby moving a lens to a lens position having a highest frequency component in order to achieve focus.
Accordingly, the speed of auto-focus processing by the contrast detection method depends on a cycle speed in which the image data is read out from the CCD (read out cycle). If the read out cycle is short, the time required for auto-focus processing becomes short, accordingly.
There are two typical ways to drive the CCD for performing auto-focus processing by the contrast detection method. One is to drive the CCD in a same way as the case when a general through image is displayed. The other is to drive the CCD in such a way so that the electrical charges loaded to the pixels of the CCD are read only from a specified area (specified are being smaller than the entire CCD area).
When the CCD is driven in the same way as the case when the general through image is displayed, the time necessary for performing auto-focus processing is not shortened. However, there is an advantage in that the through image of a subject can be displayed in real time even when auto-focus processing is performed (see FIG. 2A).
On the other hand, when the CCD is driven in such a way that the image data is read only from the specified area, it is possible to reduce the amount of pixel data to be transferred. This makes it possible to increase the frame rate and to shorten the time necessary for auto-focus processing. On the contrary, there is a disadvantage in that the through image of the subject cannot be displayed when auto-focus processing is being performed (see FIG. 2B).
For this reason, there has emerged a new technique that achieves both the advantages of reducing the auto-focus processing time and displaying through image when performing auto-focus processing is being performed. In this technique, the CCD is driven in such a way so that only the image data read out from the specified area of the CCD is displayed in real time when auto-focus processing is performed. On the other hand, images obtained in the past are displayed for area other than the specified area (for example, see Unexamined Japanese Patent Publication No. 2003-333409).